Cranial Specialization and Locomotor Habit in the Lagomorpha

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Abstract
SYNOPSIS. The skull of modern leporid lagomorphs is structurally specialized to permit significant intracranial movement. This appears to be the first recorded instance of organized cranial kinesis in mammals. A well-defined intracranial joint encircles the braincase between the posterior occipito-otic complex and the remainder of the cranium. It arises from the retention and elaboration of a zone of patency found in neonates. The joint allows the much heavier anterior region of the cranium ($ mandible) to move relative to the posterior region which is stabilized by its muscular attachment to the neck. It is hypothesized that the kinesis functions as a shock-absorbing mechanism to minimize the jarring effects (possibly on the visual apparatus) of the large impulsive loads associated with running and jumping. The dissipation of kinetic energy through controlled skull deformation may be augmented by the hydraulic displacement of intracranial blood through a specialized system of venous channels and sinuses located within the fenestrated posterior regions of the cranium. Both biomechanical considerations and behavioral observations indicate that the relatively massive external ears of cursorial leporids may play a vital role in cranial kinesis by helping to “reset’ the kinetic mechanism between loading cycles. The evolutionary origin of this specialized cranium, it is suggested, may be associated with the development of a cursorial, saltatory locomotor habit in lagomorphs. Cranial kinesis is lacking in such noncursorial forms as the living pikas (=Ochotonidae) and certain primitive fossil leporids of North America (e.g., Paleolagus, Megalagus).

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